Republicans Donald Trump and Ted Cruz on Sunday jockeyed for the crucial conservative vote before the first U.S. presidential nominating contest in Iowa on Monday, sparring over health care, voting records and a Supreme Court ruling that legalized gay marriage.

Trump, the billionaire New York real estate developer who has previously said same-sex marriage was the law of the land, told Fox News that if elected, he would seek to appoint judges who could overturn the ruling.

"If I’m elected, I would be very strong in putting certain judges on the bench who maybe could change things. But we have a long way to go," Trump said, saying the U.S. Supreme Court should have let states decide the issue for themselves.

Trump has maintained a narrow lead over Cruz, the Texas senator, ahead of the caucuses in the mid-western state, the first real test for candidates in a year in which outsiders have upended establishment politics.

The influential Des Moines Register/Bloomberg Politics Iowa poll released on Saturday showed a tight race, with Trump receiving 28 percent of the support of likely Iowa caucus-goers and Cruz 23 percent. Senator Marco Rubio of Florida was overwhelmingly the third choice, at 15 percent.

Cruz hammered away at Trump on Sunday, saying in an interview with NBC News’ “Meet the Press,” voters “can’t get burned again” and must back a rock-solid conservative.

“They’re looking for a consistent conservative. Someone they can trust to be a fiscal conservative, a social conservative, a national security conservative,” Cruz said.

Cruz told the "Fox News Sunday" program Trump was seeking to expand President Barack Obama's landmark health care law and essentially shared the position of Democratic front-runner Hillary Clinton on a key bellwether issue for conservatives.

Ten Republicans and three Democrats are campaigning in Iowa for their parties' nominations, but much of the focus has been on the fight between Cruz and Trump and their uneasy relationship with the Republican establishment.

Trump described Cruz as a "nasty guy" who had not scored key endorsements in the Senate, and would have trouble leading change in Washington.

"Nobody likes him and... you can't run a country that way...it will be a total mess. It will be worse gridlock than you have right now." Trump said in an interview with ABC's "This Week."

Cruz continued his tear into Trump on CNN's State of the Union, calling Trump's decision not to appear onstage at Thursday night's Republican debate "a real mistake," and likening the billionaire businessman's view's to those of Obama and self-described Democratic socialist Bernie Sanders.

"I think it was because he didn't want his record questioned," Cruz said.

"I get that it is unpleasant to have your record subject to scrutiny - to be potentially criticized - but this is a job interview."

In the Democratic competition, Clinton held a slight edge over Sanders, the senator from Vermont, 45 percent to 42 percent, according to Saturday's poll.

On Monday, Iowans will gather in homes, gymnasiums, libraries, taverns and even grain elevators for caucuses to select their favorite for the Democratic and Republican presidential nominations. When they are finished, the race will take on a new dynamic and several candidates could drop out altogether.